A smoke detector with an external sampling volume operates by emitting light outside its housing and detecting light scattered back into the housing by smoke particles located within the sampling volume. Smoke detectors with an external sampling volume have several important benefits over conventional ionization and photoelectric smoke detectors. First, by eliminating the internal sensing chamber and the slow accumulation of smoke particles therein, the lag time between when a threshold smoke density is reached outside the detector and when the smoke detector responds is substantially eliminated. This increases the Available Safe Egress Time (ASET), the time available for occupants to safely evacuate a building before the fire renders evacuation impossible. Second, by obviating the need for the entry of smoke particles into the housing, the entirety of the smoke detector may be mounted within an opening in a ceiling or wall, such that there is minimal protrusion outward from the surface; such flush mounting of the smoke detector creates an aesthetically pleasing appearance. Third, the smoke-detecting element may be fully tested. In conventional ionization and photoelectric smoke detectors equipped with a test feature, the testing mechanism tests the electrical circuitry only, but in smoke detectors with an external sampling volume, the operation of the smoke-detecting element may be tested by inserting an object into the sampling volume.
Despite these benefits, smoke detectors with an external sampling volume have not been widely deployed. One reason is the difficulty of these smoke detectors to isolate the signal generated by the scattered light from the signal generated by ambient light, especially when there is a change in the ambient light level. Another reason is the difficulty these smoke detectors have distinguishing smoke particles from nuisance particles or other objects. The ambiguity in both cases may lead to false alarms when a nuisance source is present or a lack of response when a fire source is present.
Accordingly, there is a need for smoke detectors with an external sampling volume, and related detection techniques, which can reject the influence of ambient light and distinguish smoke particles from nuisance particles and objects.